1. Field of Invention
This invention relates to a lamp with at least one base, and relates especially to a lamp with at least one base wherein the base is fixed by means of an adhesive to a sealing portion sealing an external lead which supplies energy to a light emission element in a light-emitting tube.
2. Description of Related Art
To prevent that the external lead supplying energy to the light emission element arranged in the light-emitting tube reaches a high temperature in the condition of being exposed to the outside air and oxidizes, hitherto the periphery was covered with an antioxidant layer. Japanese Patent Application Publication JP-A-2006-525637 and corresponding U.S Patent Application Publication 2006/0232211 A1 and Japanese Patent Application Publication JP-A-2004-139959 and corresponding U.S. Pat. No. 7,095,175 B2 disclose such means.
In FIG. 5, the respective schematical configuration is shown. In this drawing, a filament lamp is shown. In a light-emitting tube 2 of this lamp 1, a filament 3 is arranged as the light emission element. Both ends of this element are connected via internal leads 5 to metal foils 6. External leads 7 are connected to the metal foils 6, and said light emission element (filament) 3 is supplied with energy via the external leads 7. The internal leads 5, the metal foils 6 and the external leads 7 are sealed by sealing portions 4.
Further, the periphery of the external leads 7 is covered by an antioxidant layer 8, by means of which an oxidation of the external lead 7 is prevented. For this antioxidant layer, a material with a lower oxidation potential than that of the material constituting the external lead is chosen. If, for example, the external material is formed by molybdenum, the antioxidant layer is made, for example, from tin, lead, copper, silver, cadmium, platinum or gold.
By means of covering the outer surface of the external lead with an antioxidant layer, the antioxidant layer becomes exposed to the outside air and the antioxidant layer oxidizes beforehand. But, because the antioxidant layer is constituted by a material with a lower oxidation potential than that of the material of the external lead, the corrosion speed of the oxidation is lower than that of the external lead and the expansion of the volume with regard to the elapsed time is smaller as compared to an oxidation of the external lead. Therefore, the antioxidant layer covering the external lead is effective in suppressing cracks of the sealing portion.
But, when this kind of lamp is mounted in a lighting tool, a base is mounted to the sealing portion, and in general, the base is fixed to the sealing portion by using an adhesive. In FIG. 6, this configuration is shown. An antioxidant layer 8 is applied to the external lead 7, and a terminal 12 is mounted at the outer end of the lead. Further, a base 10 is fixed to the sealing portion 4 by means of an adhesive 11. That is, the adhesive 11 is filled into the base 10 containing the outer periphery of the external lead 7, and this base 10 is fixed to the sealing portion. For the adhesive 11, an inorganic adhesive (the components of which being, for example, 49% Al2O3, at most 1% LiO2, at most 1% Na2O, 31% SiO2, and H2O forming the rest) is used.
With lamps having such a configuration with at least one base, it is frequently observed that the time until cracks are generated in the sealing portion is shorter in comparison to lamps with a structure having no base. As to the cause of this phenomenon, the present inventors have found out in experiments that in the condition of the adhesive contacting the external lead, the alkali (Li, Na) contained in the adhesive reacts with the antioxidant layer at the outer surface of the external lead and corrodes the antioxidant layer so that the ability of the antioxidant layer to suppress an oxidation is decreased. When the ability of the antioxidant layer to suppress the oxidation is decreased, the oxidation of the external lead is promoted and the external lead expands. When the external lead embedded in the sealing portion expands, the sealing portion is pressed by the expanding external lead and cracks are generated which often entail breakages.